If you're like me, and don't have access to the BIW (Body In White) tools, this "Junction" surface can easily be modeled with the basic GSD tools.
1. start with 3 sketches (the ends of the extrusions). I broke the arcs into two, 90° segments and made them Output Features on each sketch
2. Intersect two of the sketch planes, and use this line as the center axis of a Revolute surface. Repeat for the other side.
3. Extrude the other segment for the straight back side
4. add a Fill surface between the 3 edges